Header compressed packet receiving apparatus and method

ABSTRACT

When an error is detected in a received header, in estimating reference information while assuming an error in a packet receiving interval, a header is decompressed using at least one value of another candidate sequence numbers used in correcting an erroneous sequence number, corresponding to a time that elapses between previously receiving a packet correctly and receiving a current packet and to the packet receiving internal. It is thereby possible to increase a possibility of estimating the reference information correctly and suppress the number of discarded packets at a receiving side, while suppressing increases in introduced processing amount in data transmission with header compression.

TECHNICAL FIELD

The present invention relates to a header compressed packet receivingapparatus and method, and more particularly, to a header compressedpacket receiving apparatus and method in techniques for compressingheader information assigned in each protocol to transmit in transmittingtransmission data using a plurality of transmission protocols.

BACKGROUND ART

Among protocols (communication procedures) used in transmitting data areInternet Protocol (IP), User Data Protocol (UDP:RFC768) and Real TimeTransport Protocol (RTP:RFC1889), and in data transmission it is generalto combine these protocols to use. These protocols are standardized byan organization called IETF (Internet Engineering Task Force).

These protocols have different roles in data transmission. IP assigns anaddress on the internet. UDP assigns a port number in a terminal anderror detecting code to detect whether data has an error in itscontents. RTP assigns time information (Time Stamp: hereinafterabbreviated as TS) on transmission data and sequence number (hereinafterabbreviated as SN) of the data. In data transmission, as illustrated inFIG. 1, header information to be assigned in each protocol is added tothe data.

Specifically, TS/SN is added to a payload in RTP packetizing processing,a port number is added to the RTP packet in UDP packetizing processing,and an IP address is added to the UDP packet in IP packetizingprocessing.

Among header information added to a payload, there is some kind ofinformation that does not need to be transmitted always and is enough tobe transmitted once or at times. When a method is used where such headerinformation is transmitted once only for the first time and is nottransmitted subsequently, or is transmitted only at proper timing, thetransmission efficiency is improved. Such a technique is called a headercompression technique. In particular, compression of IP/UDP/RTP headersis recommended as RFC2508 in IETF.

Further, there is a technique which has transmission error resistance,increases a compression rate as compared to the conventional headercompression, and is called a robust header compression technique. Thesemethods have been proposed in IETF. In the robust header technique, thecompression is implemented by a method as described below.

Data such as an IP address and port number that is constant duringcommunications is transmitted only for the first time, whereby asubsequent data amount is reduced.

With respect to TS, when the correlation with increases in SN is high,only SN is transmitted, and TS is calculated from an increase amount ofSN of a last arrived packet, whereby the data amount of the header isfurther reduced.

With respect to SN, only insignificant bits are transmitted instead oftransmitting all the bits each time, and when a carry occurs to asignificant bit, all SN is transmitted.

In this way, in the robust header compression, a header of received datais decompressed referring to the header information of a previouslytransmitted packet. The information thus referred to is called referenceinformation.

In order to implement header compression as described above, two headertypes (IR: Initial Refresh IR-DYN: IR-Dynamic) are prepared. Each headertype is shown in FIG. 2. Almost the header types contain error detectingcode (CRC: Cyclic Redundancy Check), and a receiving side is capable ofchecking whether the decompressed header is correct.

IR is a header type (including CID, profile, CRC, static chain andoptional dynamic chain) for transmitting constant information asdescribed above, and is often transmitted at the time of startingcommunications. IR-DYN is a header type for transmitting information(for example, TS and SN in RTP and check sum in UDP) that variesdynamically without compressing, and is transmitted to re-acquiresynchronization when the reference information is not coincident betweena transmitting side and receiving side, for example, due to transmissionerror. The compressed packet is a header type for compressinginformation that varies dynamically based on the reference informationto transmit.

There are some types of compressed packets. Principal types are three asdescribed below. Type 0 is a header composed of 1 byte includinginsignificant 4 bits of SN and CRC, and has the highest compressionrate. Type 1 is a header composed of 2 bytes including insignificant 4bits of SN, insignificant 6 bits of TS and CRC, and is used when thetime information changes. Type 2 is a header composed of 3 bytesincluding insignificant 6 bits of SN, insignificant 5 bits and CRC.

Since a code indicative of a type of the header is assigned to abeginning of each header, the receiving side is capable of identifyingthe header uniquely. In FIG. 2, CID in IR is called a context ID, and isan ID assigned individually to a packet transmitted on a single channel.The profile is information indicative of a header to be compressed.Static chain is the constant information as described previously. Forexample, when D is 1, it is possible to transmit a dynamic chain as anoption.

IR-DYN is the same as IR except the dynamic chain, where the informationthat varies dynamically is transmitted. M in types 1 to 2 is a markerbit contained in an RTP packet header. The bit is a flag which is “1” ina packet containing a last unit (1 frame when data is an image) of somemeaning of media data. X indicates the presence or absence of expansioninformation. Bit sequences of “0” and “1” except the foregoing aredetermined in the specification indicating a type of the packet. Inaddition, while the number of bits of each element varies in thespecification with used mode, brief descriptions are given herein.

Specific transmission procedures with no transmission error occurringwill be described below with reference to FIG. 3. Specific descriptionsto understand a general idea of the header compression are omittedherein.

In FIG. 3, IR is first transmitted, so that the transmitting side sharesreference information with the receiving side. In second transmission,IR-DYN is transmitted, so that the transmitting side shares dynamicallyvarying information with the receiving side. In third transmission, inorder to transmit a header with SN of 1 (SN=1), only insignificant 4bits of SN are transmitted using type 0. The receiving side decompressesreceived SN with the insignificant 4 bits into original SN with 2 bytes,and decompresses SN to TS (it is herein assumed that TS is decompressedfrom SN readily using the linear relationship between SN and TS)

It is checked whether the header of the received packet has an errorfrom the decomposed header and received CRC. The header with no error isconsidered as being received correctly, and is provided to the upperprotocol layer (IP). Subsequently, the transmitting side transmitspackets respectively with SN of 2 to 15 in similar procedures, while thereceiving side receives the packet in similar procedures.

When SN is 16 (SN=16), type 0 of packet is insufficient in the number ofbits of SN to transmit, and therefore, cannot transmit SN. Thusexceeding a range to represent due to a small number of bits is calledwraparound. When the wraparound occurs, type 2 is selected andtransmitted which is a header type enabling transmission of 6 bits ofSN. The receiving side decompresses insignificant 6 bits of SN to entireSN of 2 bytes.

A case where a transmission error occurs will be described withreference to FIG. 4. FIG. 4 illustrates the case where the procedures isthe same as in FIG. 3 and a transmission error occurs in SN of 5 (SN=5).When an error occurs in the header with SN of 5, performing CRC on thedecompressed header detects an error.

In this case, the packet containing the header is discarded. In asubsequent packet with SN of 6 (SN=6), the header is decompressed usingthe reference information of the header with SN of 4, since thereference information of the header with SN of 4 does not differ fromthe reference information of the header with SN of 5, the header of thepacket with SN of 6 can be decompressed accurately. In other words, evenwhen a middle packet is lost due to the error, as long as the referenceinformation is not updated, it only happens that the error packet isdiscarded, and the effects are not imposed on subsequent packets. It isthereby possible to implement header compression with resistance toerror.

As described above, with respect to a packet loss when the referenceinformation does not change, effects of an error does not propagate,while effects of an error propagate when the error occurs in a packetwhere the wraparound occurs.

FIG. 5 illustrates an error with respect to a packet where thewraparound occurs. FIG. 5 shows the same transmission and receptionprocedures as in FIG. 3, and illustrates a case where an error occurs ina packet with SN of 16 where the wraparound occurs. In this case, sincenext SN of 17 is transmitted in type 0 of packet, only significant 4bits of SN are transmitted. Since SN of 16 is discarded, the receivingside tries to decompress SN using the reference information at the timeof receiving SN of 15.

The reference information of significant bits of SN should be “0000 00000001” normally at the time of receiving SN of 16, but is lost due to theerror, and is still “0000 0000 0000”. Since insignificant 4 bits of SNof 17 are “0001”, decompressed SN becomes “0000 0000 0000 0001” i.e.“1”. Since “17” should be decompressed originally, in this case CRCdetects an error, despite a packet with SN of 17 being receivedproperly. In such a case, the receiving side understands that thewraparound occurs, and is capable of performing decompression on theassumption that significant bits that is the reference information of SNare “0000 0000 0001”. There is a case that such processing on thereceiving side enables decompression.

The above example describes the case where a single packet with thewraparound generated therein is lost, but actually, it sometimes happensthat depending on transmission path conditions, packets does not reachthe receiving side temporarily, and a plurality of successive packets islost. The receiving side assumes the number of times the wraparound hasoccurred to perform decompression. In other words, the receiving sideestimates the number of times the wraparound has occurred from the time(hereinafter referred to as an elapsed time) which elapses between acorrectly received last packet and a current packet, and assumessignificant bits that is the reference information of SN to decompress.

As the method of estimating the number of times the wraparound hasoccurred, the number of received packets is estimated from the elapsedtime and packet receiving interval, and further, the referenceinformation is estimated to perform decompression. This method iseffective when the packet receiving interval is constant, but when thepacket receiving interval varies and the elapsed time is increased, theerror is increased and it is not possible to estimate correct referenceinformation, resulting in a problem that a possibility of decompressinga received packet correctly becomes extremely low.

For example, when a packet receiving interval that a receiving siderecognizes is 10 ms and an elapsed time is 1 second, hundred packets arereceived during the elapsed time. Since the wraparound occurs once every16 times, in this case it is judged that the wraparound has occurred 6times (100/16=6.25). FIG. 6 illustrates such a situation.

However, when the actual packet receiving interval is 11 ms, aboutninety packets are received, in this case the number of wraparound timesis 5 (90/16=5.625) which differs from the number of wraparound timesestimated by the receiving side, and since it is not possible toestimate correct reference information, there is a problem that thepossibility of decompressing a received packet correctly becomesextremely low.

DISCLOSURE OF INVENTION

It is an object of the present invention to provide a header compressedpacket receiving apparatus and method capable of increasing apossibility of decompressing a received packet correctly, andsuppressing the number of discarded packets, while suppressing increasesin introduced processing amount in data transmission with headercompression.

The object is achieved by calculating the number of candidate sequencenumbers used in correcting a packet corresponding to a packet receivinginterval and the time (elapsed time) which elapses between lastreceiving a correct packet and receiving a current packet that has anerror when a header has the error, decompressing headers correspondingto the calculated number of sequence numbers, performing error detectionon each of the decompressed headers, and when finding only a singleheader with no error, decompressing the received packet using theheader.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating a configuration of packet data;

FIG. 2 is a diagram illustrating types of packets in robust headercompression;

FIG. 3 is a sequence diagram illustrating data transmission with notransmission error in robust header compression;

FIG. 4 is a sequence diagram to explain a state where a transmissionerror does not propagate when the transmission error occurs in datatransmission in robust header compression;

FIG. 5 is a sequence diagram to explain a state where a transmissionerror propagates when the transmission error occurs in data transmissionin robust header compression;

FIG. 6 is a sequence diagram to explain a state where packets are lostsuccessively when a transmission error occurs in data transmission inrobust header compression;

FIG. 7 is a block diagram illustrating a configuration of a radiocommunication apparatus provided with a header compressed packetreceiving apparatus according to one embodiment of the presentinvention; and

FIG. 8 is a block diagram illustrating a configuration of the headercompressed receiving apparatus according the one embodiment of thepresent invention.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will be described below withreference to accompanying drawings.

(First Embodiment) FIG. 7 is a block diagram illustrating aconfiguration of a radio communication apparatus provided with a headercompressed packet receiving apparatus according to this embodiment ofthe present invention. FIG. 8 is a block diagram illustrating aconfiguration of the header compressed receiving apparatus according tothis embodiment of the present invention.

While FIG. 7 illustrates only a configuration as a receiving side of theradio communication apparatus for the explanation, it is assumed thatthe apparatus has a configuration as a transmitting side.

A radio signal transmitted from an apparatus of a transmitting side isreceived in radio reception section 102 via antenna 101. Radio receptionsection 102 performs predetermined radio reception processing (forexample, such as downconverting, A/D conversion and demodulation) on theradio signal, and outputs packet data (demodulated packet data)subjected to the radio reception processing to header compressed packetdecompressing section 103.

The packet data includes compressed header information, and headercompressed packet decompressing section 103 decompresses the headerinformation. The packet data with decompressed header information isoutput to IP packet processing section 104. IP packet processing section104 extracts a UDP packet according to an IP header of the decompressedheader information. The UDP packet is output to UDP packet processingsection 105.

UDP packet processing section 105 extracts an RTP packet from the UDPpacket. The RTP packet is output to RTP packet processing section 106.RTP packet processing section 106 extracts media data from the RTPpacket. The media data is output to media decoding section 107.

Media decoding section 107 performs decoding corresponding to media onthe media data. Since the media data is coded speech data and/or imagedata, when the media data is coded speech data, speech data decoding isperformed, and image data decoding is performed when the media data iscoded image data. The decoded media data is output to media outputsection 108. Media output section 108 reproduces the media data using anappropriate device (such as a speaker and monitor).

Header compressed packet decompressing section 103 has packet receivingsection 201 that receives packet data, packet information dividingsection 202 that divides the packet data into a payload portion andheader portion, header decompressing section 203 that decompresses thecompressed header, first error detecting section 205 and second errordetecting section 210 that detect whether a decompressed header has anerror, header correcting section 209 that corrects a header in which anerror is detected, candidate SN determining section 208 that determinescandidate sequence (SN) numbers from an elapsed time, packet receivinginterval and reference information, selector 211 that outputs a headeraccording to an error detection result, reference information memory 204that stores the reference information, packet counter 207 that countsthe number of packets received per unit time, timer 206, and packetconfiguring section 212 that configures a packet using the headerportion and payload portion.

The operation of the header compressed packet receiving apparatus withthe above configuration will be described below.

A packet received in packet receiving section 201 in header compressedpacket decompressing section 103 is divided into a payload portion andheader portion in packet information dividing section 202. Headerdecompressing section 203 decompresses the header using the compressedheader information and the latest reference information stored inreference information memory 204. The decompressed header is output tofirst error detecting section 205 with an error detecting code added tothe compressed header.

First error detecting section 205 determines whether or not thedecompressed header has an error using the error detecting code, andoutputs the result to selector 211. When an error is detected, theheader is output to header correcting section 209 so as to correct theheader.

Herein, header correction will be described. The header correction isperformed by determining the number of candidate headers correspondingto the elapsed time between a last received correct packet and areceived current packet, and performing error detection on each of thecandidate headers. When a correct header is obtained, the header isoutput to selector 211.

Specifically, when first error detecting section 205 detects an error,the detection result is output to selector 211 and timer 206. Timer 206outputs an elapsed time to candidate SN determining section 208.Further, when the detection result in first error detecting section 205is correct, time 206 resets the elapsed time. Therefore, an output oftimer 206 when an error is detected is equal to the elapsed time.

Meanwhile, the packet data output from packet receiving section 201 isoutput to packet counter 207. Packet counter 207 counts the number ofpackets received per unit time. Then, packet counter 207 outputs apacket receiving interval obtained from the number of packets receivedper unit time to candidate SN determining section 208.

Candidate SN determining section 208 determines candidate sequencenumbers from the elapsed time from timer 206, packet receiving intervalfrom packet counter 207 and reference information from referenceinformation memory 204. The method of determining sequence numbers willbe described later specifically.

In the above descriptions, assuming the packet type for transmittinginsignificant 4 bits, the wraparound occurs once every 16 packets.Accordingly, from the elapsed time and packet receiving interval, it ispossible to obtain the number of packets received during the elapsedtime. Dividing the number of packets by 16 (the number of packets duringwhich the wraparound occurs once) calculates the number of times thewraparound has occurred.

Next, assuming a case where an error occurs at a predetermined rateduring the packet receiving interval, the number of wraparound times iscalculated using the packet receiving interval including the error. Asan example, a case of including the error of 10% is assumed. The numbersof wraparound times are calculated from 10% decreased and increasedpacket receiving intervals. The number of candidates is obtained from adifference between the numbers and the number of wraparound timescalculated from the packet receiving interval with no error.

Descriptions are given using equations. Assuming an elapsed time is T(ms), packet receiving interval is Pt (ms), and the number of wraparoundtimes calculated using the error of +10% or −10% with respect to Pt isrespectively W+or W−:W+=int(T/(1.1×Pt×16))W−=int(T/(0.9×Pt×16))

Accordingly, the number of candidate sequence numbers is (W−)−(W+)+1.Then, assuming the sequence number in reference information memory 204is RefSN and a candidate sequence number is CanSN(n), followingequations are obtained:CanSN(1)=RefSN+((W+)<<4)CanSN(2)=RefSN+((W+)+1)<<4)CanSN(n)=RefSN+((W−)<<4)(n=((W−)−(W+)+1))

While the above-mentioned example describes the case of the error of10%, an error may be determined corresponding to characteristics of thenetwork actually, and the present invention is not limited particularlyin determining the error.

Specifically, when the elapsed time is 1 second and the packet receivinginterval is 10 ms,

$\begin{matrix}{{W\text{+}}\; = {{int}\mspace{11mu}\left( {1000/\left( {1.1 \times 10 \times 16} \right)} \right.}} \\{= {{{int}\mspace{11mu}(5.68)} = 5}} \\{{W\text{-}}\; = {{int}\mspace{11mu}\left( {1000/\left( {0.9 \times 10 \times 16} \right)} \right.}} \\{= {{i\; n\; t\mspace{11mu}(6.94)} = 6}}\end{matrix}$

Herein, when SN in reference information memory 204 is 15, followingequations are obtained:CanSN(1)=15+5<<4=85CanSN(2)=15+6<<4=111

Accordingly, candidate SN determining section 208 outputs the abovevalues to header correcting section 209. Header correcting section 209decompresses and corrects the header using part of SN of the receivedcompressed header and the candidate sequence numbers, and outputs thedecompressed header to second error detecting section 210. Second errordetecting section 210 performs error detection on the decompressedheader. The error detection result is output to selector 211.

The error detection processing is performed corresponding to the numberof candidates determined in candidate SN determining section 208. Whenthe detection result in first error detecting section 205 is good,selector 211 outputs the header obtained in header decompressing section203 to packet configuring section 212 and to reference informationmemory 204.

When the detection results in first error detecting section 205 are NGand there is only one result with no error among detection resultsobtained in second error detecting section 210, the header with no erroris output to packet configuring section 212 and to reference informationmemory 204. Meanwhile, when all the detection results obtained in seconderror detecting section 210 are NG, any data is not output to packetconfiguring section 212 and reference information memory 204. Similarly,also when a plurality of detection results is good (a plurality ofheaders has no error) among the detection results obtained in seconderror detecting section 210, any data is not output to packetconfiguring section 212 and reference information memory 204.

Packet configuring section 212 combines the header information outputfrom selector 211 and the payload data to obtain packet data, andoutputs the packet data to IP packet processing section 104. The headerinformation output from selector 211 is stored in reference informationmemory 204.

In this way, in header compressed packet reception according to thisembodiment, the number of candidate sequence numbers is at least onewhich is used in correction when a transmission error occurs. The numberof candidate sequence numbers is determined based on a packet receivinginterval. Therefore, corresponding to the packet receiving interval, thenumber of candidate sequence numbers used in correction varies.Accordingly, even when the packet receiving interval varies and anelapsed time is increased, the number of candidate sequence numbers usedin correction is increased corresponding to the interval, and it isthereby possible to increase the possibility of decompressing a receivedpacket correctly and to suppress the number of discarded packets.

Further, the number of candidates is determined corresponding to theelapsed time, the number of candidate sequence numbers used incorrection thus varies, and it is possible to suppress increases inredundant processing amount for header correction.

The header compressed packet receiving apparatus according to thepresent invention is capable of being mounted on a communicationterminal apparatus such as a radio reception terminal apparatus andradio transmission/reception terminal apparatus. It is thereby possibleto improve reception performance in transmission of header compressedpacket.

The present invention is not limited to the above-mentioned embodiment,and capable of being carried into practice with various modificationsthereof. For example, specific numerals used in the above-mentionedembodiment are not limited thereto, and capable of being carried intopractice with various modifications thereof.

Further, while the above-mentioned embodiment describes the presentinvention as an apparatus of receiving header compressed packets, thepresent invention maybe implemented by software. In other words, it maybe possible that a program of implementing the method of the presentinvention is stored in a writable storage medium such as a ROM, and thestored program is processed by a CPU. Furthermore, it may be possible toread the software from the storage medium to implement in a computer.Configuring the header compressed packet receiving apparatus of thepresent invention by software delivers the same effectiveness as that inthe apparatus configured by hardware. Moreover, it is made possible toreadily achieve the method of receiving header compressed packets asdescribed above by microcomputer and personal computer.

The above-mentioned embodiment describes the case of providing the firsterror detecting section that detects an error of a header subjected toheader decompression and the second error detecting section that detectsan error of a header subjected to header correction. However, it may bepossible to configure a section that serves as the first and seconderror detecting sections. It is thereby possible to reduce the hardwaresize.

As is apparent from the foregoing, according to the present invention,in techniques of receiving header compressed packets, one or morecandidate sequence numbers used in correction are determined when atransmission error occurs, and it is thereby possible to increase thepossibility of decompressing a received packet correctly. Further, thenumber of candidates is determined corresponding to an elapsed time, andit is thereby possible to suppress increases in redundant processingamount for header correction.

This application is based on the Japanese Patent Application No.2001-301846 filed on Sep. 28, 2001, entire content of which is expresslyincorporated by reference herein.

INDUSTRIAL APPLICABILITY

The present invention relates to a header compressed packet receivingapparatus and method, and more particularly, is suitable for use in aheader compressed packet receiving apparatus and method in techniquesfor compressing header information assigned in each protocol to transmitin transmitting transmission data using a plurality of transmissionprotocols.

1. A header compressed packet receiving apparatus comprising: areceiving section that receives a packet with a compressed headercontaining part of a sequence number; a header decompressing sectionthat decompresses compressed header information to obtain originalheader information using the compressed header information and referenceinformation beforehand received; an error detecting section that detectsan error of a sequence number in a decompressed header; a candidatesequence number determining section that determines at least onecandidate sequence number used in correcting a header, corresponding tothe part of the sequence number contained in the compressed header, atime that elapses between previously receiving a packet correctly andreceiving a current packet, and a packet receiving interval determinedfrom the number of received packets per unit time when an error isdetected in a sequence number; and a header correcting section thatdecompresses and corrects a header in which the error is detected usingthe candidate sequence number determined in the candidate sequencenumber determining section.
 2. The header compressed packet receivingapparatus according to claim 1, wherein the candidate sequence numberdetermining section calculates the number of wraparounds for a case withno error using the elapsed time and the packet receiving interval,calculates the number of wraparounds for a case with an error using theelapsed time and the packet receiving interval with an error, anddetermines a same number of candidate sequence numbers as a differencebetween the number of wraparounds for the case with no error and thenumber of wraparounds for the case with an error.
 3. The headercompressed packet receiving apparatus according to claim 1, wherein theerror detecting section detects an error of the header corrected in theheader correcting section.
 4. A communication terminal apparatus havinga header compressed packet receiving apparatus, the header compressedpacket receiving apparatus comprising: a receiving section that receivesa packet with a compressed header containing part of a sequence number;a header decompressing section that decompresses compressed headerinformation to obtain original header information from the compressedheader information and reference information beforehand received; anerror detecting section that detects an error of a sequence number in adecompressed header; a candidate sequence number determining sectionthat determines at least one candidate sequence number used incorrecting a header, corresponding to the part of the sequence numbercontained in the compressed header, a time that elapses betweenpreviously receiving a packet correctly and receiving a current packet,and a packet receiving interval determined from the number of receivedpackets per unit time when an error is detected in a sequence number;and a header correcting section that decompresses and corrects a headerin which the error is detected using the candidate sequence numberdetermined in the candidate sequence number determining section.
 5. Aheader compressed packet receiving method comprising: receiving a packetwith a compressed header containing part of a sequence number;decompressing compressed header information to obtain original headerinformation using the compressed header information and referenceinformation beforehand received; detecting an error of a sequence numberin a decompressed header; determining at least one candidate sequencenumber used in correcting a header, corresponding to the part of thesequence number contained in the compressed header, a time that elapsesbetween previously receiving a packet correctly and receiving a currentpacket, and a packet receiving interval determined from the number ofreceived packets per unit time when an error is detected in a sequencenumber; and decompressing a header in which the error is detected usingthe determined candidate sequence number.
 6. The header compressedpacket receiving method according to claim 5, wherein determining thecandidate sequence number comprises: calculating the number ofwraparounds for a case with no error using the elapsed time and thepacket receiving interval; calculating the number of wraparounds for acase with an error using the elapsed time and the packet receivinginterval with an error; and determining a same number of candidatesequence numbers as a difference between the number of wraparounds forthe case with no error and the number of wraparounds for the case withan error.
 7. A header compressed packet receiving apparatus comprising:a receiving section that receives a packet with a compressed headercontaining part of a sequence number; a header decompressing sectionthat decompresses compressed header information to obtain originalheader information from the compressed header information and referenceinformation beforehand received; an error detecting section that detectsan error of a sequence number in a decompressed header; a candidatesequence number determining section that calculates the number ofwraparounds, corresponding to a time that elapses between previouslyreceiving a packet correctly and receiving a current packet and a packetreceiving interval, when an error is detected in a sequence number, anddetermines at least one candidate sequence number used in correcting aheader based on the calculated number of wraparounds and the part of thesequence number contained in the compressed header; and a headercorrecting section that decompresses and corrects a header in which theerror is detected using the candidate sequence number determined in thecandidate sequence number determining section.
 8. The header compressedpacket receiving apparatus according to claim 7, wherein the candidatesequence number determining section calculates the number of wraparoundsfor a case with no error using the elapsed time and the packet receivinginterval with no error, calculates the number of wraparounds for a casewith an error using the elapsed time and the packet receiving intervalwith an error, and determines the candidate sequence number based on thenumber of wraparounds for the case with no error and the number ofwraparounds for the case with an error.
 9. The header compressed packetreceiving apparatus according to claim 8, wherein the candidate sequencenumber determining section determines a same number of candidatesequence numbers as a difference between the number of wraparounds forthe case with no error and the number of wraparounds for the case withan error.
 10. A header compressed packet receiving method comprising:receiving a packet with a compressed header containing part of asequence number; decompressing compressed header information to obtainoriginal header information using the compressed header information andreference information beforehand received; detecting an error of asequence number in the decompressed header; calculating the number ofwraparounds corresponding to a time that elapses between previouslyreceiving a packet correctly and receiving a current packet and a packetreceiving interval, when an error is detected in a sequence number,determining at least one candidate sequence number used in correcting aheader corresponding to the calculated number of wraparounds and thepart of the sequence number contained in the compressed header; anddecompressing a header in which the error is detected using thedetermined candidate sequence number.
 11. The header compressed packetreceiving method according to claim 10, wherein determining thecandidate sequence number comprises: calculating the number ofwraparounds for a case with no error using the elapsed time and thepacket receiving interval with no error; calculating the number ofwraparounds for a case with an error using the elapsed time and thepacket receiving interval with an error; and determining a same numberof candidate sequence numbers as a difference between the number ofwraparounds for the case with no error and the number of wraparounds forthe case with an error.
 12. The header compressed packet receivingmethod according to claim 11, wherein determining the candidate sequencenumber comprises determining a same number of candidate sequence numbersas a difference between the number of wraparounds for the case with noerror and the number of wraparounds for the case with an error.